The present invention generally relates to a contact type image sensor, and in particularly to a contact type image sensor which can be suitably applied to a facsimile machine, a digital copying machine, an optical character reader, an electronic blackboard apparatus and the like.
A conventional optical reader which has an image sensor and may be used for a facsimile machine, is shown in FIG. 1. Referring to FIG. 1, the illustrated optical reader includes an image sensor 1 of a metal oxide semiconductor (MOS) type or a charge-coupled device (CCD) type, which is produced by integrated circuit techniques. A document 2 is irradiated by a fluorescence lamp 3. Light emitted from the fluorescence lamp 3 is reflected on the document 2. Reflected light passes through a reduction optical lens 4, and forms an image on the image sensor 1. The image formed on the image sensor 1 is a reduced image. The reduced image is subjected to photoelectric conversion in the image sensor 1. Then, the image sensor 1 generates an electric signal corresponding to the reduced image.
The above optical reader utilizes the reduction optical lens 4. For this reason, in a case where a reduction factor of the optical lens 4 is increased, an optical length between the document 2 and the image sensor 1 is increased. This prevents the miniaturization of the optical reader. Additionally, the resolving power or resolution of a peripheral portion of the reduced image formed on the image sensor 1 is degraded due to optical characteristics of the optical lens 4. Further, the signal-to-noise ratio (S/N ratio) of the peripheral image portion is reduced, because of a smaller amount of light which passes through a peripheral portion of the optical lens 4.
In order to overcome the above disadvantages, an optical reader as shown in FIG. 2 has been proposed. A contact type image sensor 5 has a size identical to a width of the document 2. The document 2 is irradiated by light which is emitted from the fluorescence lamp 3 or an LED array and which passes through the image sensor 5. The light is reflected on the document 2, and the reflected light is received by the image sensor 5.
FIG. 3 shows an elevational cross section of the image sensor 5 shown in FIG. 2. FIG. 4 is a enlarged front view of the image sensor 5. An opaque metallic film 7, in which a light window 17a is formed, is formed on top of a transparent insulating substrate 6. A transparent insulating film 8 is formed on the opaque metallic film 7 and an exposed surface portion of the transparent insulating substrate 6. Lower electrodes 9 each having a light window 17b are formed on the transparent insulating film 8. A photoelectric conversion layer 10 is formed on each of the lower electrodes 9. A transparent conductive film 11 is formed on each of the photoelectric conversion elements 10. An interlayer insulating film 12 is formed as shown in FIG. 3, and through holes 28 are formed therein so that portions of the surface of the transparent conductive film 11 are exposed. Upper extension electrodes 13 which extend in a predetermined direction, are formed as shown in FIGS. 3 and 4. The upper extension electrodes 13 are kept in electric contact with the corresponding transparent conductive films 11. Finally a passivation film 14 is formed on the entire surface, and then a transparent wear-resistant plate 16 is fixed on the passivation film 14 by an adhesive layer 15.
The light emitted from the fluorescence lamp 3 passes through the light windows 17a an 17b, and is then reflected o the document 2 which is made to slide on the transparent wear-resistant plate 16. The reflected light passes through the transparent conductive film 11, and then enters the photoelectric conversion layers 10. An electric signal which corresponds to the received light, can be read out across the lower electrode 9 and the upper extension electrode 13. The above structure makes i possible to considerably reduce the optical distance between the document 2 and the image sensor 5, compared with the structure of FIG. 1 using the reduction optical lens 4. Therefore, a smaller size of the optical reader is obtainable. Additionally the image sensor 5 has the size which corresponds to that of the document 1. Therefore, the resolution of a peripheral portion of the document 2 can be prevented from being decreased.
The fluorescence lamp 3 must be located on the side of a rear (bottom) face of the image sensor 2, because the document 2 is placed so as to make contact with a front (top) face of the image sensor 5. Therefore, it is required for providing the opaque metallic film 7 in order to prevent the light emitted from the fluorescence lamp 3 from directly entering the photoelectric conversion elements 10. However, it should be noted that a stray or parastic capacitance is formed by the opaque metallic film 7 and the lower electrodes 9, between which the transparent insulating film 8 which is interposed. The stray capacitance becomes great in the case of a longitudinal image sensor. The stray capacitance is a main factor in reducing an output signal of the image sensor, increasing noise and crosstalk. Thus, sensor characteristics are degraded due to the stray capacitance. Additionally, since an opposing area of the opaque metallic film 7 and the corresponding lower electrode 9 is great, the ratio of occurrence of short-circuit due to a pin hole which may be formed in the transparent insulating film 8 is high, and correspondingly the yield rate of production of image sensors is low.